Test specific error in super-private-access-invalid.case (#2355)

Without declaring #x in the class body, the thrown error could have been
about the undeclared private name rather than about the SuperProperty

src/class-elements/super-private-access-invalid.case

@@ -23,6 +23,8 @@ features: [class-methods-private]
 extends B
 
 //- elements
+#x() {}
+
 method() {
   super.#x();
 }

test/language/expressions/class/elements/syntax/early-errors/super-private-access-invalid.js

@@ -28,6 +28,8 @@ $DONOTEVALUATE();
 
 var C = class extends B
 {
+  #x() {}
+
   method() {
     super.#x();
   }

test/language/statements/class/elements/syntax/early-errors/super-private-access-invalid.js

@@ -28,6 +28,8 @@ $DONOTEVALUATE();
 
 class C extends B
 {
+  #x() {}
+
   method() {
     super.#x();
   }